define correspondence between 0/1 and particular values of
voltage or current

basic element is a switch: device whereby one
electrical signal can be used to control another

switches can be used to perform logical functions (and, or,
not)

Time and frequency

Time is measured in seconds and in fractions of a second:

millisecond (ms): 0.001 second

microsecond (us): 0.000 001 second

nanosecond (ns): 0.000 000 001 second

picoseconds (ps): 0.000 000 000 001 second

For a repetitive phenomenon, the rate at which it repeats is the frequency,
measured in cycles per second or Hertz. For higher
frequencies we have

1 kilohertz (kHz) = 1 000 cycles / second

1 megahertz (MHz) = 1 000 000 cycles / second

1 gigahertz (GHz) = 1 000 000 000 cycles / second

There is a reciprocal relationship between frequency and the time
for a cycle (the 'period'):

frequency = 1 / clock period

clock period = 1 / frequency

For example, if a CPU operates at 100 Hz, its "clock cycle" is 0.01
second = 10 ms; if it operates at 100 MHz, its clock cycle is 0.000
000 01 second = 10 ns.

Switching elements and computer generations

(text, sec. 1.10 (on CD))

Relays were used in the earliest equipment: electronic
accounting machines, and early computers made in the 1930's
and early 1940's at Bell
Labs and by Zuse in
Germany. Relays are mechanical and hence relatively slow --- their
response time is measured in milliseconds.

Vacuum tubeswere dominant about 1945 to 1960 ('first generation of
computers'). The first general purpose computer was the ENIAC (1946):
18,000
tubes,
20 10-digit accumulators, 100 kHz clock, 200 microsecond add time,
200 KW power. Tubes were much faster than relays (because they had
no mechanical moving parts), but they were bulky, required high
power, and had a relatively short life (crucial because of the
large number of components).

Discrete transistors
('second generation computers') were used in computers starting
around 1960. They took less power , were smaller, and had a longer
lifetime than vacuum tubes.

Integrated circuits ('third generation computers') were
created by fabricating several transistors on a single chip,
allowing computers to be made smaller. IC's were introduced in the
late 1960's, and gradually increased their level of integration
(number of transistors on a chip).

Integrated circuits are made by building up a series of layers on
top of a silicon substrate. The patterns in the individual
layers are made by photolithography,
projecting patterns onto the silicon wafer.

Very large scale integration ('fourth generation
computers') represented the ability to put more and more
transistors on a chip, until an entire processor (a microprocessor)
could be fabricated on a single chip. Initially VLSI was used to
make personal computers (Apple II - 1977; IBM PC - 1981); now all
computers are made from VLSI.

Technology Trends

We continue to learn, at a steady pace, how to fabricate smaller and
smaller transistors. This allows for

a higher level of integration (currently, hundreds of millions
of gates on a chip)

The cost of a chip has remained (very roughly) constant, so
price/performance has been rapidly decreasing. How should this extra
circuitry and extra performance be used? We shall return to this
issue after discussing processor design.